I'm reading on Java I/O streams and I'm confused on the correct definition associated with them.
A stream is a concept, but it's not that strict, that just only one description would be correct.
An I/O Stream represents an input source or an output destination. A stream can represent many different kinds of sources and destinations, including disk files, devices, other programs, and memory arrays. Streams support many different kinds of data, including simple bytes, primitive data types, localized characters, and objects. Some streams simply pass on data; others manipulate and transform the data in useful ways.
No matter how they work internally, all streams present the same simple model to programs that use them: A stream is a sequence of data.
(Side note: In crypto, there's e.g. a difference between stream and block ciphers, where a stream cipher is something that does not know (in a very general sense) anything about the future, while a block cipher knows its (maximum) size in advance and the sizes of all coming blocks.)